Printed circuit boards (PCBs) are specifically designed to support electronic components, and facilitate the communication of electrical signals. As PCBs have evolved, the complexity of the electronic components and the complexity of the signal transmission structures have also evolved considerably. To accommodate the complex circuit design typically involved, modern day circuit boards are multi-layer structures, having multiple communication paths extending between hundreds of different components.
In common day PCB design, there has been an increased demand for high-speed communication capabilities. This typically involves the ability to provide a high-speed or high-frequency connection between two or more mounted components, with signals being carried by communication paths extending through the circuit board structure. Further, these communication paths may extend for relatively short distances or may extend longer distances, depending upon the nature of the circuit board and the environment within which the board is used. In addition, a number of these communication paths must traverse several layers in a circuit board, thus adding further complexity to the communication paths. As mentioned above, high-speed signal transmission is also commonly demanded, typically involving signals with a frequency range of 3 to 56 gigabits per second (Gbps), or even higher. Operating at this speed produces several complications, and creates a need to closely examine signal losses throughout the PCB.
As can be appreciated, when high speed signals, operating at a speed of 3 gigabits per second (Gbps) or higher are moving across the PCB, the various effects of most circuit board structures can potentially produce inefficiency, and create undesirable effects. In many cases, this may include signal attenuation or signal losses which are unacceptable. This is especially true when complex high speed circuit board design is involved.
As known by those involved with the design of printed circuit boards, electrical signals are carried between components using various types and lengths of signal traces. Due to the number of components involved, and the multiple signals that must be communicated between these components, current circuit board design uses various layers of the board structure to accommodate this function. The space on the various layers can be used for multiple signal traces, depending upon the space requirements. The various layers are then separated by a dielectric material to contain signals within the desired layer and along the desired signal trace.
When a communication path in a printed circuit board needs to traverse from one board layer to another layer, it travels through a drilled and plated hole called a ‘via’. While this provides an efficient communication path, the electrical impedance of this via will often differ significantly from the electrical impedance of the signal trace. This difference in impedance tends to degrade the electrical quality of the entire communication path. In manufacturing the PCB, a small circular pad of metal typically remains on each of the intervening layers and surrounds the hole. This creates a structure that is helpful in the PCB lamination process, but has no functional purpose for the circuit involved (i.e. it creates non-functional pads). Stated differently, this non-functional pad has no electrical function, but helps to facilitate board construction and reliability. In current PCB design practice, the inner non-functional pads are either all left in place, or all removed, without respect to the actual electrical characteristics of the path.